Despite continued advances in magnetic resonance (MR) instrumentation, imperfections in the magnetic field evolution still hamper numerous MR procedures. Field perturbations are caused by a range of mechanisms, including eddy currents, limited gradient bandwidth, and heating effects. Often such errors can be addressed by means of signal processing. However, to do so these errors need to be accurately known. Reproducible field perturbations can be determined approximately by preparatory measurements. Alternatively, it has been proposed to monitor the relevant field evolution directly during each actual scan, using appropriately designed magnetic field probes.
EP 1 582 886 A1 discloses a method of MR imaging wherein additional data are acquired, during execution of a MR sequence, from at least one monitoring magnetic field probe positioned in the vicinity of or surrounding an object of interest. The magnetic field probes exploit the presence of a strong magnetic field needed for the MR measurements; accordingly, they are based upon magnetic resonance in a small sample volume of an MR active substance. Upon MR excitation of the object of interest, there is a concomitant MR excitation of the substance within the probe, the resonance frequency of which is proportional to the magnitude of the local magnetic field.
As discussed in EP 1582886 A1, magnetic resonance in the probe sample can be based on nuclear magnetic resonance (NMR), in which case the probe sample shall contain a NMR active nuclear species such as 1H, 13C, 17O, 19F or 31P. Magnetic resonance imaging (MRI) is mostly based on 1H. As is well known, the choice of the nucleus for probing is subject to several considerations, and, based on the chosen nucleus, the choice of the substance used as the sample in a magnetic field probe is also subject to several considerations. According to EP 1582886 A1, normal water (1H2O) is a preferred probe substance based on 1H whereas suitable substances based on 19F include hexafluorobenzene and trifluoromethlysulfonate. It is also mentioned in EP 1582886 A1 to add suitable relaxation agents so as to modify relaxation times in the sample as needed.
An improved NMR-based magnetic field probe has been described in De Zanche N, Barmet C, Meier D, Pruessmann K. NMR probes for magnetic field monitoring during MRI. Proceedings 13th Scientific Meeting, International Society for Magnetic Resonance in Medicine; 2005, May, p 791.
Magnetic field monitoring with nuclear magnetic resonance (NMR) based field probes is also a powerful tool for hardware and sequence development, and quality assurance in MRI as well as for monitoring magnetic fields in general. During the measurements, the temperature of the NMR field probe can vary due to different effects (e.g., change in ambient conditions, radiative or conductive heating and cooling), potentially leading to inaccurate measurement values.